This feature requires version 3.8 or later of Unbabel for Salesforce Service Cloud
Unless any configuration is implemented using this feature, the default behaviour of the Unbabel composer is to populate the To address for the reply with the email address from the sender of the last email message present in the case feed. This configuration allows you to override that rule and user a case field (including custom fields) to popular one or more values into the To address field.
To set up a custom value:
1 - Head to the Unbabel SC Configurations menu
2 - Expand the Email Composer Configuration section and click New Configuration
3 - Under Field to override, select Available To Addresses from the dropdown list
4 - Add the Case field API name to the last field of the configuration item and click save